ABSTRACT: Context: Beyond providing temporary staffing, National Health Service Corps (NHSC) clinicians are believed by some observers to contribute to the long-term growth of the non-NHSC physician workforce of the communities where they serve; others worry that NHSC clinicians compete with and impede the supply of other local physicians. Purpose: To assess long-term changes in the non-NHSC primary care physician workforce of rural underserved counties that have received NHSC staffing support relative to workforce changes in underserved counties without NHSC support. Methods: Using data from the American Medical Association and NHSC, we compared changes from 1981 to 2001 in non-NHSC primary care physician to population ratios in 2 subsets of rural whole-county health professional shortage areas: (1) 141 counties staffed by NHSC physicians, nurse practitioners, and/or physician assistants during the early 1980s and for many of the years since and (2) all 142 rural health professional shortage area counties that had no NHSC clinicians from 1979 through 2001. Findings: From 1981 to 2001, counties staffed by NHSC clinicians experienced a mean increase of 1.4 non-NHSC primary care physicians per 10,000 population, compared to a smaller, 0.57 mean increase in counties without NHSC clinicians. The finding of greater non-NHSC primary care physician to population mean ratio increase in NHSC-supported counties remained significant after adjusting for baseline county demographics and health care resources (P < .001). The estimated number of “extra” non-NHSC physicians in NHSC-supported counties in 2001 attributable to the NHSC was 294 additional physicians for the 141 supported counties, or 2 extra physicians, on average, for each NHSC-supported county. Over the 20 years, more NHSC-supported counties saw their non-NHSC primary care workforces grow to more than 1 physician per 3,500 persons, but no more NHSC-supported than nonsupported counties lost their health professional shortage area designations.Conclusions: These data suggest that the NHSC contributed positively to the non-NHSC primary care physician workforce in the rural underserved counties where its clinicians worked during the 1980s and 1990s.